When a furnace of the kind specified is operated to raise the temperature of the bodies of molten glass in the heating zones by the passage of electric current therethrough, and the body of glass in the intervening zone by conductive transference of heat to the ideal operating temperature, or a temperature in the region thereof, withdrawal of glass is effected from positions distributed over a wide area of the lower part of the bodies of molten glass and the withdrawn glass is replaced by generally descending glass in the furnace chamber, itself replaced by melting from the underside of a blanket of solid state glass making material or batch lying on the surface of the molten glass bodies.
One of the problems encountered in a glass melting furnace of the kind specified is that for a given glass composition and for a given throughput (i.e. tonnage to be withdrawn from the furnace over a given period) the glass in the furnace chamber is ideally maintained at or in the region of a specific temperature, and undergoes heating in the chamber for, or in the region of, a specific residence time. It will thus be appreciated that for a given throughput the residence time is determined by the size of the chamber.
In many instances, it is required to supply molten glass at variable rates, depending upon the nature of the glass utilisation process for which the glass is required. Since it is not practical to alter the volume of the furnace chamber, variable demand for the supply of molten glass results in the glass supplied at different times having undergone different periods of residence in the furnace chamber. In general, the size of the furnace chamber will be determined by the maximum throughput which it is required to handle, to ensure that the glass arriving at the point of use is in a satisfactorily refined condition. Thus, any reduction in the rate of throughput will result in the glass having a longer residence time in the furnace chamber than is ideal. When the residence time increases due to the reduced input, various undesirable effects occur.
Thus, if the temperature of the glass in the furnace is not reduced, there will be a reduction in the thickness of the blanket of batch covering the bodies of molten glass in the furnace chamber. The heat insulating effect achieved by the presence of the batch blanket over part or all of the surface of the chamber is reduced or lost, especially if at one or more places it becomes molten prematurely throughout its thickness.
To prevent this, for throughputs less than the maximum for which the furnace is designed to handle, the temperature of the body of molten glass in the furnace chamber requires to be reduced.
If the temperature is reduced, the viscosity of the glass is lowered and the pattern of flow of glass towards the outlet tends to be disturbed. In particular, there is an increased tendency for portions of glass contained in the molten body of glass occupying the intervening zone and which lies adjacent to and above the withdrawal flow path to become entrained in the withdrawal flow, whereas at the ideal operating temperature this would not be the case. There is, in consequence, a risk that insufficiently refined glass, or glass containing particles of solid state batch material, may be drawn into the withdrawal flow path especially from a region adjacent to the surface of the glass and in the vicinity of the outlet.
In our U.S. Pat. No. 4,000,360 there is suggested, as a means of overcoming or reducing this problem, the expedient of electrically heating the body of molten glass locally above said withdrawal flow path in the vicinity of the outlet, to produce an upward convective pressure to counteract the downward pull exerted on such glass by the entrainment effect of flow along the withdrawal path. The electrical heating is effected by the provision of auxiliary electrode means in the vicinity of the outlet and an auxiliary power supply circuit is provided for connection to said auxiliary electrode means for the feed of electrical current thereto.
In this manner, some current will flow through the body of molten glass in the intervening zone above and across the withdrawal flow path locally in the vicinity of the outlet to produce an upward convective pressure to counteract the downward pull exerted on such glass by the entrainment effects of the flow along the withdrawal path. However, it has been found that, in certain circumstances, the fact that the intervening zone as a whole is not heated may produce disadvantageous effects. In particular where a large temperature differential exists between the heating zones and the intervening zone, convective currents may be produced which cause partially melted or unrefined glass to be pulled into the withdrawal flow path at regions spaced laterally from any such local heating which may be effected in the vicinity of the outlet. Such partially melted or unrefined glass will then be entrained in the stream of glass being withdrawn from the furnace chamber, and will pass beneath any such local heating means without being brought to the surface and will accordingly pass through the outlet.